Latex based coatings, such as paints, stains, adhesives, and printing inks, are being increasingly used because of their safety, economy, ease of application, and ease of clean-up. Aqueous coatings such as latex paints, dry quickly compared to solvent coatings such as alkyd paints. Because of the faster drying time of aqueous coatings relative to solvent coatings, a second coat can be applied in a short time, for example, from about 3 to about 5 hours. However, because of the fast drying nature of aqueous coatings, there is often not enough time to rebrush over the freshly coated wet surface to improve its appearance or to apply additional paint onto the freshly coated wet surface without causing defects such as brush marks, loss of gloss, or lap lines in the final dried coating. A "lap" as used herein, refers to an area on a substrate where additional coating is applied onto a portion of a previously coated, but still wet, adjacent substrate area.
In the case of paint, it is desirable to seamlessly join up the edges of a wet painted area with newly added fresh paint without any visible lap showing in the dried coating. "Lapping" or "to lap" refers to the ability to do this seamless joining of edges without leaving a visible "lap" line. As used herein, "open time" or "wet-edge time" refers to the time that a coating remains workable, after it has been applied to a substrate, to allow for rebrushing or "melting in" of the newly applied coating at the lap, without resulting in the above defects. Open time is a more apparent problem in cases where other than a flat or dull surface appearance is desired, such as with sheen and gloss paints, because a glossy appearance makes the surface defects more visible.
The drying characteristics of aqueous coatings, such as dry time, flow and gloss of the dried coating, are not easily controlled because they are dependent on how fast the water in the coating evaporates. During the drying of the aqueous coating, as water evaporates, polymer particles in the latex binder deform and fuse together to form a continuous film. This process, known as "film formation" or "coalescence", is irreversible since the addition of water onto the dried or drying coating will not redisperse the polymer particles. Once coalescence occurs, it is impossible to rebrush the aqueous coating and lap into a previously coated area. The drying behavior of aqueous coatings depends on the temperature, humidity and air velocity at the time of application and thereafter. For example, coatings can dry almost instantaneously under hot, dry, or windy conditions, leading to poor lapping characteristics and short open times. In addition, these conditions also tend to impair the quality of film formation, which can result in cracks, poor flow, low gloss, and poor adhesion to the substrate.
The conventional practice for addressing the quick drying nature of aqueous coatings has been to add substantial levels of water-soluble solvents, for example, 10 to 20 percent or more based on total liquid content, to such coatings to increase the open time. These techniques are discussed by M. D. Andrews, "Influence of Ethylene and Propylene Glycols on Drying Characteristics of Latex Paints", Journal of Paint Technology, vol. 46, page 40 (1974); D. A. Sullivan, "Water and Solvent Evaporation from Latex and Latex Paint Films", Journal or Paint Technology, vol. 47, page 60 (1975); and C. R. Martens, Waterborne Coatings, Van Nostrand Reinhold, page 153 (1981). These references discuss the use of short chain water-soluble alcohols and glycols as cosolvents in aqueous coatings to aid coalescence of the latex particles, improve levelling, and prolong open time. However, these cosolvents negate the intrinsic advantages of aqueous coatings, such as safety, low tack, low odor, and low pollution.
In addition to enhancing open time of latex coatings, such water-soluble solvents also provide freeze-thaw stability to the latex-based coatings. One is faced then with the problem of preparing latex based binders and coatings which do not utilize such solvents or which utilize significantly reduced amounts of such solvents, yet which exhibit open-time and freeze-thaw stability which are as good as or better than those of latex binders and coatings which utilize such solvents at levels which are effective to provide freeze-thaw-stable latex binders and coating compositions.